Biocides are substances that kill or inhibit the growth of microorganisms such as bacteria, fungi and algae. Biocidal chemicals include chlorinated hydrocarbons, organometallics, halogen-releasing compounds, metallic salts, quaternary ammonium compounds, phenolics and organic sulfur compounds.
Exemplary of organic sulfur compounds are compounds based on an isothiazolinone or isothiazolothione structure. The biocidal activity of these compounds is effected by inactivation of essential enzymes of microbial metabolism which require sulfhydryl groups for activity. These enzymes include phosphoenolpyruvate transphosphorase and a number of dehydrogenases. The thio moiety of the isothiazolinone or isothiazolothione compounds reacts with the free sulfhydryl groups of an enzyme to form a disulfide bond between the enzyme molecule and the isothiazolinone or isothiazolothione molecule rendering the sulfhydryl unavailable for interaction with substrate or effector molecules.
Isothiazolinone and isothiazolothione biocides have found widespread use as latex preservatives. Most latex emulsions are water based and are prone to microbial attack. Biocides are typically added to the finished latex after all processing is completed to protect the latex from microbial attack.
Manufacture of latex by emulsion polymerization requires certain free-radical generating chemicals to initiate polymerization. At elevated temperatures, these radical-generating compounds undergo homolytic decomposition to form active free radicals which participate in the propagation reaction which makes polymerization possible. Several different types of thermal initiators are currently in use; exemplary are azo compounds and peroxy compounds such as 2,2'-azobisisobutyronitrile, peroxydisulfate ion, benzoyl peroxide and t-butyl hydroperoxide. Alternatively, polymerization can be initiated with a redox initiator system containing an oxidizing and a reducing agent. The redox systems also generate free radicals; exemplary are ferrous ion with hydrogen peroxide, sodium formaldehyde sulfoxylate, sodium thiosulfate and sodium metabisulfite.
During the manufacture of latex, the polymerization process is rarely allowed to proceed to 100% monomer to polymer conversion due to the negative effect of excessive chain branching and molecular weight distribution on polymer structure when conversions approach 100%. Post-polymerization latex therefore contains a substantial amount of monomer.
Recently, latex manufacturers have increased their efforts to reduce the monomer content of finished latex to increase latex emulsion stability. Redox systems, such as those described above or t-butyl hydroperoxide and a bisulfite salt or thermal initiators as described above are often added to latex after polymerization to reduce the monomer content. The systems act to generate free radicals and activate residual monomers which are then bonded by an addition reaction to the polymer bringing the free monomer concentration to around 0.1% or less. The redox system components or other radical-generating chemicals which are added to the latex, either to initiate polymerization or to reduce post-polymerization monomer content are largely consumed by these reactions. However, finished latex still contains some residual amount of these radical-generating chemicals. It has been found that the residual radical-generating chemicals react with the active ingredients of biocide formulations and destroy their biocidal activity.
Biocides which are widely used as latex preservatives include PROXEL.RTM. GXL, having an active ingredient of 1,2-benzisothiazolin-3-one (BIT), PROMEXAL.RTM. W50, having an active ingredient of 2-methyl-4,5-trimethylene-4-isothiazolin-3-one, both available from Zeneca Inc., and KATHON.RTM. LX, a blend of 5-chloro-2-methyl-4-isothiazolin-3-one and 2-methyl-4-isothiazolin-3-one active ingredients, available from Rohm and Haas Company. Decrease in biocidal activity results in a latex that is not sufficiently protected from microbial attack and in reduced cost efficacy of the biocide. Thus, it is desirable to stabilize the biocides used in products and processes where free radicals are generated by stabilizing the biocides against free-radical degradation.
While organic stabilizers for isothiazolinones such as orthoesters and epoxy compounds have been disclosed in U.S. Pat. Nos. 4,906,274 and 5,127,934, respectively, these patents are directed specifically to protection of isothiazolinones from molecules containing nucleophilic groups. Nucleophiles are ions or molecules which can donate a pair or electrons to form a covalent bond with another atom. A molecule containing a nucleophilic group is electron-rich and makes that molecule more likely to donate a pair of electrons for bond formation. In contrast, free radicals are molecular fragments which have one or more unpaired electrons and in a reaction, seek to acquire a single additional electron. Thus, free radicals are not considered to be nucleophiles and the isothiazolinone stabilizers of the prior art would not protect the compounds from attack and degradation by free radicals.
Copper salt stabilizers of oil soluble, water insoluble isothiazolinones have been disclosed in U.S. Pat. No. 5,108,500. However, the salts disclosed are not free-radical scavengers.
Clearly, a need exists for compounds which can stabilize isothiazolinone or isothiazolothione biocides, i.e., protect them from free-radical attack and degradation. Such stabilizing compounds would enable the use of decreased amounts of isothiazolinone or isothiazolothione biocides while maintaining biocidal efficacy and improve the cost-effectiveness of the biocidal compounds as preservatives.